In thermal ink-jet printing, droplets of ink are collectively emitted from a plurality of drop ejectors in a printhead, in accordance with digital instructions, to create a desired image on a sheet. In most current designs of an ink-jet printer, the printhead is relatively small compared to the size of a sheet on which an image is printed, and the printhead is caused to reciprocate relative to the sheet, with the sheet being indexed a short distance with every pass, or following a certain number of passes, of the printhead.
Because of the relatively small size of a commercial printhead relative to a sheet (a linear array being typically 2 cm or shorter in length), any small advantage in improving the speed of outputting sheets is desirable. One well-known technique familiar in the art of ink-jet printing is called "white space skipping." With white space skipping, any "white" areas in the image (i.e., portions of an image in which no ink is intended to be placed) are pre-identified from the original image data, and can thus be skipped entirely by the moving printhead. For example, assuming the printhead reciprocates across the short direction relative to a letter-sized sheet printing out a portrait-oriented image, any white space at the top or the bottom of the image (such as the margins of a letter) can simply be passed through the apparatus quickly without the printhead passing over these areas at all. Similarly, if there is a sufficiently large white space on one particular side of the image (such as if there are wide right or left margins), the printhead can be caused to change direction before reaching the end of the sheet. In short, if it is known in advance that no portion of the image is to be printed in a certain place, the printhead need not pass over that part of the sheet having only white area, or, alternately, the printhead can pass over the white space at a speed higher than would be suitable if the printhead were depositing ink in the area; for purposes of the claims hereinbelow, either technique can fall under the rubric of "skipping."
White space skipping is made possible in the context of printing an image based on original data derived from a computer, because this digital data will inherently hold "clues" which the control system of the printer can use to detect large white spaces in the image to be printed. For example, page description languages, such as PostScript.TM., facsimile protocols such as Group 3, or in some cases JPEG-compressed data, can contain short codes, or markers, which indicate that a certain area within the image to be printed is white; these markers can be exploited to cause the printhead to skip certain areas. A problem arises, however, if the basic digital printing apparatus is used as part of a digital ink-jet copier, in which the image to be printed is derived not from original computer data, but rather from an original hard-copy image which is scanned from a platen or other input scanning device, such as part of a facsimile. The problem with such original image data is that, unlike computer-originated images, the data inherently includes no "clues" to facilitate white-space skipping when the copies are printed.
An object of the present invention is to provide a system which enables the speed advantages of white-spaced skipping in an ink-jet-based copier.